Effects of H2/NH3/CH4 oxygen-enriched combustion and flow rate on heating performance and emissions in a walking-beam reheating furnace

The use of ammonia and hydrogen as carbon-free fuels is an effective strategy for reducing CO2 emissions in reheating furnaces. However, nitrogen-containing fuels, particularly ammonia, may increase NO emissions. Therefore, combustion optimization plays a critical role in mitigating CO2 and NO formation. This paper numerically investigates two operating strategies in a full-scale walking-beam reheating furnace fired with a 10% H2/10% NH3/80% CH4 fuel blend by volume: oxygen-enriched combustion at constant fuel flow and oxygen-enriched combustion combined with fuel-flow reduction. Seven operating cases were analyzed. Cases 1–4 evaluate oxygen enrichment at a constant fuel flow, with the oxygen mole fraction increased from 21 to 35 vol%. Cases 5–7 evaluate the combined effect of oxygen enrichment and fuel-flow reduction, with the fuel-flow reduced to 80% and 60%. Compared with Case 1, the flue-gas velocity decreased by up to 66% in Case 7, while radiative heat transfer to the slabs increased by 26.2% in Case 4. Oxygen enrichment increased the outlet CO2 mass fraction. However, when combined with fuel-flow reduction, the total CO2 mass flow rate decreased by 41.7% in Case 6 compared with Case 1. NO emissions were also substantially reduced, reaching a maximum reduction in outlet NO mass-flow rate of 93% in Case 7 compared with Case 1. Furnace thermal efficiency improved from 53.6% in Case 1 to 70.9% in Case 7. These numerical results suggest that H2/NH3/CH4 oxygen-enriched combustion combined with fuel-flow reduction may improve energy efficiency and reduce emissions in industrial reheating furnaces under the investigated conditions.